Balluff BIS M-370-000-A02, Manual, Page: 108 / 198

MODBUS TCP INTERFACE
101
6
6.3 MODBUS TCP - HANDSHAKING
Due to the process with which commands and responses are passed between the BIS M-
626_ and the host, a handshaking procedure is used to notify the host that returning data is
available for retrieval.
Overall Length
The hands
haking process is governed by the changing of the “
Overall Length
” value within
a data packet. The Overall Length value is typically the first word (2-bytes) of a command or
response and indicates the total number of data words in the packet.
Node Input and Node Output Pages
Under the Modbus TCP protocol, host-generated data is written to a pre-defined region of the
BIS M-
626_’s own memory known as the
Node Input Page . Host-bound data generated by
the BIS M-626_, is written to a separate region of the BIS M-
626_’s memory known as the
Node Output Page (in Modbus TCP these regions of memory are called Device IDs ). Node
Input and Node Output Pages are used to temporarily hold incoming (controller-bound) and
outgoing (host-bound) data.
Output Data Ready Mask
To notify the host that new data is waiting to be retrieved from the Node Output Page, the
BIS M-626_ utilizes a separate 32-bit block of internal memory, called the
Output Data
Ready Mask .
The first bit of the 32-bit Output Data Ready Mask represents the status of the Node Output
Page. For example, the first or lowest bit (
bit 01 ) represents Node Output Page 33 - which
holds output data from Node 01.
The BIS M-626_, itself, is assigned Node 01 and thus, its corresponding Node Output Page
is 33. As noted, Node Output Page 33 is represented by the first bit (
bit 01 ) in the Output
Data Ready Mask.
Holding Registers
When writing host-bound data to Node Output Page 33, the BIS M-626_ actually places each
byte of the data packet into pre-
defined “
holding registers
” within the Node Output Page.
Note that a single holding register stores 2-bytes or one word of data. The 2-byte Overall
Length value, for example, is written to the first holding register (which is location 40001) of
the Node Output Page.
Then, as the BIS M-626_ finishes writing host-bound data to the Node Output Page, the
Overall Length value (stored at holding register 40001) will change from its default value of
0x00 to reflect the number of data words within the newly written host-bound data packet.
This change to the Overall Length value (i.e. register 40001) within the Node Output Page,
triggers the BIS M-626_ to enable (change from zero to one) bit one in the Output Data
Ready Mask. It is when bit one in the Output Data Ready Mask has become enabled, that
the host will recognize the pending data.
Finally, after the host has retrieved its pending data, the enabled bit in the Output Data
Ready Mask and the Overall Length value at holding register 40001 of the Node Output